The invention concerns a method of producing cylinders such as printing or embossing cylinders having a three-dimensionally patterned surface, by means of a lithographic method.
In this specification the term cylinder is used to include a printing cylinder, an embossing cylinder or like cylinder which is to have a three-dimensional pattern at its surface.
In one form of method of producing a cylinder with a three-dimensionally patterned surface, a radiation-sensitive layer forming the surface of the cylinder is irradiated progressively in dot-wise manner in accordance with the desired pattern by means of an exposure beam which is imaged or projected on to the layer. The layer is ablated or removed in region-wise manner and thereby the surface of the cylinder is patterned.
It is known for the surface of printing cylinders to be patterned by means of a procedure referred to as laser lithography. In that operation a metal cylinder coated with photoresist is exposed by means of a laser in dot-wise manner in accordance with the desired pattern. After the exposure operation the photoresist is washed off at the exposed locations in a chemical developer bath. The metal forming the surface of the printing cylinder is then bare at the washed-off locations. That metal is then exposed to suitable chemicals, in particular acids, in order to etch a suitable pattern, for example a print image, into the surface of the cylinder. In that method dot sizes of between about 5 and 50 .mu.m are used. With those dot sizes the operation of exposing the entire surface of a roller which is about 1 meter in length and 1 meter in periphery lasts for between about 1 and 2 hours. If it is assumed that the raster dot size of 15 .mu.m which is usually employed is used, then by calculation that gives an exposure frequency of between 500 kHz and 1 MHz which can be produced for example by means of acousto-optical modulators which are connected in the form of optical switches into the beam path of the exposure beam. If that method involves using a UV-laser with a wavelength of about 350 nm, a laser power at the focus of about 1 W is required, in which respect the printing cylinder can rotate at up to 1600 rpm during the exposure operation.
The above-outlined method can readily be used for structuring the surface of printing cylinders where structures of an order of magnitude of between 5 and 50 .mu.m are absolutely sufficient to achieve a suitable quality of image. For certain areas of use however, for example for producing surface structures which are operative by a diffraction or interference-optical effect, as are used for example for decorative or security purposes, substantially finer structures are required. For example, in the case of relief structures for security elements which vary their appearance, for example undergo a change in color, in dependence on the variation in the illumination and/or viewing angle, use is made of grating structures in which the grating constant is markedly less than 1 .mu.m. In order to be able to produce such fine structures, it would be necessary in the case of the above-discussed method to operate with dot sizes of less than 0.5 .mu.m. In order to be able to operate with such small dot sizes, it would be necessary on the one hand to position the exposure beam with the utmost degree of accuracy, which is not possible with this system and which would at any event involve a very high level of apparatus expenditure because of the extremely accurate synchronisation that would possibly be required in terms of the exposure frequency and the frequency of rotation of the cylinder. In addition, the exposure times would be shortened by a factor of between 100 and 1000 per dot with a total exposure time remaining the same, and that also could only be embodied using extremely expensive measures. Accordingly, with normal exposure times for the individual dot, the total exposure time would be increased by a factor of between 100 and 1000. In both cases the exposure method would become very expensive and thus uneconomical.
It is already known from U.S. Pat. No 5,291,317 to produce fine structures, in particular holographic grating structures which comprise a plurality of picture dots or pixels, by a procedure whereby, upon irradiation of a substrate, the individual pixels are already finely structured in themselves by using a beam-forming element. In that method however the substrate is precisely adjusted prior to any irradiation operation, that is to say to produce each pixel, and is absolutely not moved during the irradiation procedure. That mode of operation is not possible when structuring printing cylinders as the respective mass to be accelerated would be excessively great. If however the cylinder is not stopped during the exposure operation, the usual surface speeds at speeds of rotation of around 1000 rpm and with a usual exposure time of about 1 usec result in lack of sharpness and definition of the imaged or projected beam, and under some circumstances that lack of sharpness and definition is greater than the degree of fineness of the structures, for example the spacing of grating lines forming the structure. It seems therefore that the method in accordance with U.S. Pat. No 5,291,317 cannot be used for producing surface patterning comprising individual, finely structured pixels on a rotating cylinder.